1. Field of the Invention
This invention concerns liquid crystal display systems which are multiplex driven.
2. Description of the Prior Art
A matrix type liquid crystal display system is provided with electrode groups in which a large number of fine lines made of a transparent conductor such as indium-tin oxide are arranged in parallel on the inner surfaces of 2 substrates made of transparent glass which face each other. These are arranged so that they cross each other with a gap between them and have cells between them in which liquid crystal is placed. Images are formed on the surface of the liquid crystal display by applying waveforms of select-voltage level or non-select voltage level to these electrodes line by line.
Since the application of a direct current voltage to liquid crystal decomposes and degrades the liquid crystal, the liquid crystal cell drive is carried out by an alternating current voltage waveform. In practice, frame inversion is used which reverses the polarity of the drive voltage every frame,
However, display non-uniformity, such as the crosstalk phenomenon on the screen, often occurs depending on the display pattern. For this reason, the N-line inversion method was developed by alternating the current every scanning electrode N-line by making the polarity inversion cycle shorter than 1 frame. However, even driving by this type of inversion method, display non-uniformity still occurred and, with the demand for larger screens and gray scale displays, this became a great problem.
As the causes of display non-uniformity, it is considered that the electrical resistance of the electrode lines of the liquid crystal display panel, the electrostatic capacity of the liquid crystal, the dielectric anisotropy of the liquid crystal and the frequency dependence all have their effects.
FIG. 27(a) shows the data waveform outputted from a data electrode drive device. Here, if the scanning waveform outputted from the scanning electrode drive device in the 1 frame period shown in FIG. 27(a) is taken as the intermediate voltage V1 between V0 and V2 and in the inverted frame period as intermediate voltage V4 between V3 and V5, the voltage waveform actually applied to the liquid crystal cell at this time becomes as in FIG. 27(b). For reference, the scanning voltage is also shown in the same Figure. That is to say, V0 and V5 in the Figure show the voltage level (select data voltage level Vds) which designates the ON state in a pixel when the scanning electrode is not driven, and V2 and V3 show the voltage level (non-select data voltage level Vdn) which designates the OFF state in a pixel. Therefore, the voltage outputted from the data electrode drive device changes as in FIG. 27 depending on the content which it is wished to display.
Moreover, even if a square waveform such as (a) is applied from the data electrode drive device, the voltage waveform (b) actually applied to the liquid crystal is distorted and distortion occurs in the rising and falling portions. The amount of this varies by a time constant depending on the resistance of the electrode line and the electrostatic capacity of the liquid crystal. Also, since the number of polarity reversals differs depending on the display pattern, the size of the distortion varies. For this reason, the larger the distortion and the greater the number of polarity reversals, the more the effective voltage value applied to the liquid crystal reduces. When this type of reduction of the effective voltage value occurs, the select and non-select states of the liquid crystal cannot accurately be switched. Also, it results in unsatisfactory switching, and is therefore considered to be a cause of display non-uniformity.
However, since the electrode lines of the liquid crystal display panel must be formed thinly and finely in order to ensure light transmissivity and the required number of pixels, there is a limit to the reduction of this line resistance. Also, the electrostatic capacity, the dielectric anisotropy and the frequency dependence of the liquid crystal are all inherent properties and cannot be eliminated.
In Japanese Patent Laid-Open Showa 62-287226 and Japanese Patent Laid-Open Heisei 2-6921, examples have been disclosed in which the number of waveform subjected to distortion is made constant by providing a period in which the voltage applied to the liquid crystal display panel is made 0 V in every scanning period, and does not depend on the display constant. However, since no consideration has been given to the size of the distortion in these examples, not only is the effect on display non-uniformity unsatisfactory, but there are also cases when the display non-uniformity is, rather, made worse. That is to say, in these examples, there is display non-uniformity which occurs due to the reduction of the RMS voltage because the distortion gradually becomes greater from the pixels close to the electrode drive device of the liquid crystal panel to the distant pixels, and display non-uniformity caused by changes in the electrostatic capacity due to the ON/OFF switching of pixels. Thus, not only is there no effect on display non-uniformity due to the difference of the size of the distortion, but also, since, as a result, the number of times distortion occurs increases, these examples have the effect of increasing display non-uniformity.